cgen: key the str/slice arg recognizers off the checker stamp

The wwstage nodeisstr/nodeisslice recognizers were name-keyed for
every non-local shape: an indirect fn-pointer callee ((*f)() — the
errnotest #59.5 divergence, streq receiving a shifted register file)
and module-global let/const idents (path.sepstr — union-widen pushes
zero-filled len/cap) both fell to false while cstage keys off the
checker stamp unconditionally. Both recognizers now fall back to the
stamped n.type_; push and pop sites share them, so the drain stays
balanced by construction. Graduates the #59.5 errnotest pin.

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1233;
+def corpuscount: i32 = 1236;
 def errorcount: i32 = 314;
 def compilecount: i32 = 12;
-def runcount: i32 = 141;
+def runcount: i32 = 144;
 def runexitcount: i32 = 766;
-def nativecount: i32 = 2466;
-def corpushash: str = "65d3642dfb77b486f5aed23044a979bbce37c36fbbcb7a7aedae20d87c7b0e4f";
+def nativecount: i32 = 2472;
+def corpushash: str = "4c8f4a693d2d8193b15f4215610c1d070dc83493b1189af772ca09cad4a69db7";
 
 type directive = enum i32 {
 	ERROR = 0,